Some engine control strategies monitor operating parameters using exhaust gas sensors in order to control engine output. For example an engine control strategy may adjust engine torque based on a feedback air-fuel ratio. Furthermore, some engine control strategies advantageously monitor the exhaust gas sensors in order to determine functionality.
One approach attempts to monitor air-fuel sensors based on various parameters includes performing diagnostics on sensors based on the sensor's exposure to different air-fuel ratios and air amounts. See, for example, U.S. Pat. No. 6,758,185.
However, the inventors herein have recognized that this approach may have some disadvantages. For example, when the above approach is applied to a system with cylinder group specific modes of engine operation and an asymmetric air-fuel sensor configuration in the exhaust, diagnosis of sensor degradation may have larger errors due to confounded readings based on mixed air-fuel measurements as a result of each cylinder group operating at a different air-fuel ratio. Furthermore, some sensor types may produce more accurate readings in specific air-fuel ratio ranges and may have different accuracies corresponding to one another. For example, a linear exhaust gas sensor may have more accurate air-fuel ratio readings in a lean air-fuel ratio range. As another example a switching type exhaust gas sensor may have more accurate air-fuel ratio reading at stoichiometry. Therefore, diagnosis of sensor degradation in a system containing various engine modes with different air-fuel ratios and different exhaust gas sensor configurations may have increased errors depending on the type of sensor and respective range of measurement accuracy.
The above issues may be addressed by, in one example, a multi-cylinder group engine system operable in at least a first mode and a second mode, where in the first mode a first and second cylinder group combust air and fuel, and where in the second mode at least one of the first and second cylinder group combusts air and fuel and the other one of the first and second cylinder group pumps air without injected fuel, the engine system comprising: a first linear exhaust gas sensor disposed in a first exhaust passage to measure air fuel exhausted from the first cylinder group; a first switching type exhaust gas sensor disposed in a second exhaust passage to measure air-fuel exhausted from the second cylinder group; a second linear exhaust gas sensor disposed downstream of a junction between the first exhaust passage and the second exhaust passage; and a controller configured to validate a reading of the first linear exhaust gas sensor during the first mode of engine operation based on a reading of the second linear exhaust gas sensor when a lean air-fuel ratio is commanded.
In this way, air-fuel control readings may be validated in an asymmetric sensor configuration to facilitate accurate engine control throughout different modes of engine operation. In particular, a linear exhaust gas sensor may advantageously be validated based on another linear exhaust gas sensor during lean operation due to the accurate nature of linear exhaust gas sensor measurements in a lean air-fuel ratio range. Furthermore, by validating the readings of the exhaust gas sensors the engine control strategy may reduce the likelihood of inaccurate engine torque control and estimation, increased emissions and/or reduced fuel economy due to errors in air-fuel control because of sensor degradation.